A superabsorbent polymer (SAP) is a synthetic polymeric material capable of absorbing moisture at about 500 to 1000 times its own weight, and is also called a SAM (super absorbency material), an AGM (absorbent gel material), etc. Since superabsorbent polymers started to be practically applied in sanitary products, they have now been widely used not only for hygiene products such as disposable diapers for children, etc., but also for water retaining soil products for gardening, water stop materials for civil engineering and construction, sheets for raising seedlings, fresh-keeping agents for food distribution fields, and the like.
As a preparation process for such superabsorbent polymers, a process of reverse phase suspension polymerization and a process of solution polymerization have been known.
Of them, preparation of the superabsorbent polymer by reverse phase suspension polymerization is disclosed in, for example, Japanese Patent Laid-open Publication Nos. S56-161408, S57-158209, S57-198714, etc.
Further, preparation of the superabsorbent polymer by the solution polymerization further includes a thermal polymerization method in which a water-containing gel polymer is polymerized while being broken and cooled in a kneader equipped with a plurality of shafts, and a photo-polymerization method in which an aqueous solution at a high concentration is irradiated with UV rays on a belt to be polymerized and dried at the same time.
Meanwhile, the absorption rate, which is one of important physical properties of the superabsorbent polymer, is associated with surface dryness of products in contact with skin, such as diapers. Generally, the absorption rate may be improved by increasing the surface area of the superabsorbent polymer.
For example, a method of forming a porous structure on the particle surface of the superabsorbent polymer by using a foaming agent is applied. However, since it is difficult to form a sufficient amount of the porous structure by the foaming agent, there is a drawback that the absorption rate is not greatly increased.
Another example is a method of increasing the surface area by regranulating fine particles obtained during the preparation process of the superabsorbent polymer to form non-uniform porous particles. This method may be used to improve the absorption rate of the superabsorbent polymer, but there is a limitation in that centrifuge retention capacity (CRC) and absorbency under pressure (AUP) of the polymer become relatively low. That is, there is a trade-off between physical properties of the superabsorbent polymer such as absorption rate, centrifuge retention capacity, absorbency under pressure, etc. Accordingly, there is an urgent demand for a preparation method capable of improving these physical properties at the same time.